The present invention relates to a method of logging earth formations traversed by a borehole and, more particularly, to an improved method for the determination of water saturation in subsurface earth formations substantially independent of lithology utilizing electromagnetic energy propagation measurements.
The search for oil and gas has resulted in a need for a method of distinguishing directly hydrocarbons from water in subsurface earth formations. One technique suggested to make such determination relies upon the dielectric permittivity or dielectric constant of materials. Dielectric constant is a basic electrical property of matter for which hydrocarbons and water have distinctly different characteristics. Basically, the dielectric constant of any material is proportional to the electric dipole moment per unit volume. With the exception of water, there are very few materials abundantly found in nature which have permanent electric dipoles.
The primary measurements for determining dielectric constant of material are electromagnetic energy propagation time and attenuation rate. The electromagnetic energy propagation time of water differs considerable from that of oil. Such a measurement provides a determination of water saturation in subsurface earth formations which is substantially independent of water salinity. Experimentation has found that the loss-free electromagnetic energy propagation time for oil to be approximately 4.9 nanoseconds per meter which relates to a relative dielectric permittivity of 2.2. Likewise, the loss-free electromagnetic energy propagation time for water has been found to be from between 25-30 nanoseconds per meter relating to a relative dielectric permittivity from between 56-80. Thus, a measurement of the dielectric permittivity of the earth materials provide a method of directly estimating formation water saturation substantially independent of water salinity variations.
One difficulty encountered in estimating water saturation in the vicinity of a borehole from electromagnetic energy propagation time measurement is the so called "matrix effect". Matrix effect described is a major unknown which is the value of the electromagnetic energy propagation time value of the complex lithologies. For such complex lithologies it is common to determine the electromagnetic propagation time value empirically from the matrix grain density and an estimate of the lithologic components as derived from additional logging measurements. These additional logging measurements require the expenditure of time and money. Additionally, empirical estimates have been found to be inaccurate in the presence of gas or other light hydrocarbons in the formations.
These and other disadvantages are overcome with the present invention by providing a method for determining the water saturation in earth formations surrounding a borehole substantially independent of complex lithologies.